This invention relates to data communication equipment or modems.
Data communication equipment (DCEs), or modems, are devices used to transmit and receive binary data over a communication channel. One category of DCEs, sometimes referred to as full-duplex modems, is capable of performing the functions of transmitting and receiving simultaneously. When the modem is transmitting and receiving simultaneously over a two-wire communication link (e.g., transmission over the switched telephone network), an echo of the transmitted signal is commonly present in the signal received from the remote modem. If the transmitted and received signals occupy the same frequency band, it is necessary to remove the echo signal, in order to reliably detect the data sent by the remote modem.
The echo signal typically has a near echo and a far echo component The near echo is generated by the imperfect hybrid couplers in the local modem and the near-end telephone central office. The far echo, on the other hand, is mainly generated by the hybrid couplers in the remote central office and the remote modem. The far echo is delayed in time relative to the near echo. When this delay can be substantial, the echo canceller is often broken into a near-echo and a far-echo canceller component which are also separated by a delay.
High-speed modems typically use bandwidth-efficient modulation schemes such as quadrature modulation. In such systems, the binary data is first mapped into a sequence of complex signal points (symbols) chosen from a constellation with a finite number of points. The real-valued transmitted signal carries information about this complex sequence.
Two-wire, full-duplex, high-speed modems, such as the standard V.32 voiceband modem specified by the CCITT, come equipped with adaptive echo cancellers which are capable of nearly eliminating the echoes of the transmitted signal. An echo canceller is typically implemented as a transversal filter which consists of a tapped-delay line, and a series of variable complex-valued tap coefficients. The inputs to the tapped-delay line are the aforementioned complex signal points. These are appropriately weighted by the tap coefficients to generate as output the real part of the weighted running sum. This represents an approximation of the received real-valued echo signal. The echoes are cancelled by subtracting this estimated echo signal from the real received signal.
Echo cancellers which are implemented as a transversal filter with a complex-input and a real output are often referred to as Nyquist echo cancellers. Nyquist echo cancellers often consist of a near canceller and a far echo canceller. One realization of Nyquist echo cancellers is described by S. Weinstein in the U.S. Pat. No. 4,131,767 (reissue Re31,253).
An echo canceller is typically trained in the absence of the remote signal, during an initialization or training period which occurs prior to data transmission. In many echo cancellers, the transversal filter is trained using the least mean-square (LMS) algorithm. In an LMS algorithm the tap coefficients are continually adjusted to remove any correlation between the complex input symbols and the residual received signal which remains after echo cancellation. However, the time required to accurately train an echo canceller in this manner can be very long, particularly in modems which employ echo cancellers with long transversal filters 10. In the past, fast training methods have been discussed for echo cancellers whose input and output are either both real- or both complex-valued. One method using an ordinary periodic chirp sequence was disclosed by T. Kamitake in IEEE Proc. of ICC'84 (pp. 360-364, May 1984, Amsterdam, Holland) in a paper entitled "Fast Start-up of an Echo Canceller in a 2-wire Full-duplex Modem". A similar method using a pseudo-random shift-register sequence was later described by V. Kanchan and E. Gibson in IEEE Trans. on ASSP (Vol. ASSP-86, No. 7, pp. 1008-1010, Jul. 1988) in a paper entitled "Measurement of Echo Path Response". These methods are not applicable to Nyquist echo cancellers with a complex input and a real output.
In IEEE Proc. of GLOBECOM'87 (pp. 1950-1954, Nov. 1987, Tokyo, Japan), J. M. Cioffi proposed a method for fast simultaneous training of both the near and the far echo cancellers of a Nyquist echo canceller in a paper entitled "A Fast Echo Canceller Initialization Method for the CCITT V.32 Modem." This method is based on the discrete Fourier transform (DFT) and uses a real periodic pseudo-noise sequence, however, it assumes a perfect Hilbert filter (transformer) in the transmitter which is generally not realizable.
When the echo path can be substantially modeled as a linear filter, a transversal filter can effectively reconstruct and substantially cancel the echo signal. However, in certain instances, the far echo may also contain a small amount of frequency offset, also referred to as phase roll, which complicates echo cancellation. Some more sophisticated echo cancellers also include phase-roll compensation circuitry which can track the phase variations in the far echo and thereby remove its detrimental effect. Typically, the phase-roll compensation circuitry includes a phase-locked loop (PLL) to acquire the phase-roll frequency and phase during the training period.